Human organic anion transporter 2 is distinct from organic anion transporters 1 and 3 with respect to transport function

Molecular basis for selective uptake and elimination of organic anions in the kidney by OAT1

In mammals, the kidney plays an essential role in maintaining blood homeostasis through the selective uptake, retention or elimination of toxins, drugs and metabolites. Organic anion transporters (OATs) are responsible for the recognition of metabolites and toxins in the nephron and their eventual urinary excretion. Inhibition of OATs is used therapeutically to improve drug efficacy and reduce nephrotoxicity. The founding member of the renal organic anion transporter family, OAT1 (also known as SLC22A6), uses the export of α-ketoglutarate (α-KG), a key intermediate in the Krebs cycle, to drive selective transport and is allosterically regulated by intracellular chloride. However, the mechanisms linking metabolite cycling, drug transport and intracellular chloride remain obscure. Here, we present cryogenic-electron microscopy structures of OAT1 bound to α-KG, the antiviral tenofovir and clinical inhibitor probenecid, used in the treatment of Gout. Complementary in vivo cellular assays explain the molecular basis for α-KG driven drug elimination and the allosteric regulation of organic anion transport in the kidney by chloride.

Hypoxic erythrocytes mediate cardioprotection through activation of soluble guanylate cyclase and release of cyclic GMP

Red blood cells (RBCs) mediate cardioprotection via nitric oxide-like bioactivity, but the signaling and the identity of any mediator released by the RBCs remains unknown. We investigated whether RBCs exposed to hypoxia release a cardioprotective mediator and explored the nature of this mediator. Perfusion of isolated hearts subjected to ischemia-reperfusion with extracellular supernatant from mouse RBCs exposed to hypoxia resulted in improved postischemic cardiac function and reduced infarct size. Hypoxia increased extracellular export of cyclic guanosine monophosphate (cGMP) from mouse RBCs, and exogenous cGMP mimicked the cardioprotection induced by the supernatant. The protection induced by hypoxic RBCs was dependent on RBC-soluble guanylate cyclase and cGMP transport and was sensitive to phosphodiesterase 5 and activated cardiomyocyte protein kinase G. Oral administration of nitrate to mice to increase nitric oxide bioactivity further enhanced the cardioprotective effect of hypoxic RBCs. In a placebo-controlled clinical trial, a clear cardioprotective, soluble guanylate cyclase-dependent effect was induced by RBCs collected from patients randomized to 5 weeks nitrate-rich diet. It is concluded that RBCs generate and export cGMP as a response to hypoxia, mediating cardioprotection via a paracrine effect. This effect can be further augmented by a simple dietary intervention, suggesting preventive and therapeutic opportunities in ischemic heart disease.

Stimulation of Erythrocyte Soluble Guanylyl Cyclase Induces cGMP Export and Cardioprotection in Type 2 Diabetes

Reduced nitric oxide (NO) bioactivity in red blood cells (RBCs) is critical for augmented myocardial ischemia-reperfusion injury in type 2 diabetes. This study identified the nature of "NO bioactivity" by stimulating the intracellular NO receptor soluble guanylyl cyclase (sGC) in RBCs. sGC stimulation in RBCs from patients with type 2 diabetes increased export of cyclic guanosine monophosphate from RBCs and activated cardiac protein kinase G, thereby attenuating ischemia-reperfusion injury. These results provide novel insight into RBC signaling by identifying cyclic guanosine monophosphate from RBC as a mediator of protection against cardiac ischemia-reperfusion injury induced by sGC stimulation in RBCs.

Probiotics, bioactive compounds and dietary patterns for the effective management of hyperuricemia: a review

Hyperuricemia is closely linked with an increased risk of developing hypertension, diabetes, renal failure and other metabolic syndromes. Probiotics, bioactive compounds and dietary patterns are safe cost-efficient ways to control hyperuricemia, whereas comprehensive reviews of their anti-hyperuricemic mechanisms are limited. This review summarizes the roles of probiotics, bioactive compounds and dietary patterns in treating hyperuricemia and critically reviews the possible mechanisms by which these interventions exert their activities. The dietary patterns are closely related to the occurrence of hyperuricemia through the indirect action of gut microbiota or the direct effects of host purine metabolism. The Mediterranean and Dietary Approaches to Stop Hypertension diets help reduce serum uric acid concentrations and thus prevent hyperuricemia. Meanwhile, probiotics alleviate hyperuricemia by ways of absorbing purine, restoring gut microbiota dysbiosis and inhibiting xanthine oxidase (XO) activity. Bioactive compounds such as polyphenols, peptides and alkaloids exert various anti-hyperuricemic effects, by regulating urate transporters, blocking the active sites of XO and inhibiting the toll-like receptor 4/nuclear factor kappa B signaling pathway and NOD-, LRR- and pyrin domain-containing protein 3 signaling pathway. This review will assist people with hyperuricemia to adopt a healthy diet and contribute to the application of natural products with anti-hyperuricemic activity.

Physiological Characterization of the Transporter-Mediated Uptake of the Reversible Male Contraceptive H2-Gamendazole Across the Blood-Testis Barrier

The blood-testis barrier (BTB) is formed by a tight network of Sertoli cells (SCs) to limit the movement of reproductive toxicants from the blood into the male genital tract. Transporters expressed at the basal membranes of SCs also influence the disposition of drugs across the BTB. The reversible, nonhormonal contraceptive, H2-gamendazole (H2-GMZ), is an indazole carboxylic acid analog that accumulates over 10 times more in the testes compared with other organs. However, the mechanism(s) by which H2-GMZ circumvents the BTB are unknown. This study describes the physiologic characteristics of the carrier-mediated process(es) that permit H2-GMZ and other analogs to penetrate SCs. Uptake studies were performed using an immortalized human SC line (hT-SerC) and liquid chromatography-tandem mass spectrometry (LC-MS/MS). Uptake of H2-GMZ and four analogs followed Michaelis-Menten transport kinetics (one analog exhibited poor penetration). H2-GMZ uptake was strongly inhibited by indomethacin, diclofenac, MK-571, and several analogs. Moreover, H2-GMZ uptake was stimulated by an acidic extracellular pH, reduced at basic pHs, and independent of extracellular Na+, K+, or Cl- levels, which are intrinsic characteristics of OATP-mediated transport. Therefore, the characteristics of H2-GMZ transport suggest that one or more OATPs may be involved. However, endogenous transporter expression in wild-type Chinese hamster ovary (CHO), Madin-Darby canine kidney (MDCK), and human embryonic kidney-293 (HEK-293) cells limited the utility of heterologous transporter expression to identify a specific OATP transporter. Altogether, characterization of the transporters involved in the flux of H2-GMZ provides insight into the selectivity of drug disposition across the human BTB to understand and overcome the pharmacokinetic and pharmacodynamic difficulties presented by this barrier. SIGNIFICANCE STATEMENT: Despite major advancements in female contraceptives, male alternatives, including vasectomy, condom usage, and physical withdrawal, are antiquated and the widespread availability of nonhormonal, reversible chemical contraceptives is nonexistent. Indazole carboxylic acid analogs such as H2-GMZ are promising new reversible, antispermatogenic drugs that are highly effective in rodents. This study characterizes the carrier-mediated processes that permit H2-GMZ and other drugs to enter Sertoli cells and the observations made here will guide the development of drugs that effectively circumvent the BTB.

Molecular Properties of Drugs Handled by Kidney OATs and Liver OATPs Revealed by Chemoinformatics and Machine Learning: Implications for Kidney and Liver Disease

In patients with liver or kidney disease, it is especially important to consider the routes of metabolism and elimination of small-molecule pharmaceuticals. Once in the blood, numerous drugs are taken up by the liver for metabolism and/or biliary elimination, or by the kidney for renal elimination. Many common drugs are organic anions. The major liver uptake transporters for organic anion drugs are organic anion transporter polypeptides (OATP1B1 or SLCO1B1; OATP1B3 or SLCO1B3), whereas in the kidney they are organic anion transporters (OAT1 or SLC22A6; OAT3 or SLC22A8). Since these particular OATPs are overwhelmingly found in the liver but not the kidney, and these OATs are overwhelmingly found in the kidney but not liver, it is possible to use chemoinformatics, machine learning (ML) and deep learning to analyze liver OATP-transported drugs versus kidney OAT-transported drugs. Our analysis of >30 quantitative physicochemical properties of OATP- and OAT-interacting drugs revealed eight properties that in combination, indicate a high propensity for interaction with "liver" transporters versus "kidney" ones based on machine learning (e.g., random forest, k-nearest neighbors) and deep-learning classification algorithms. Liver OATPs preferred drugs with greater hydrophobicity, higher complexity, and more ringed structures whereas kidney OATs preferred more polar drugs with more carboxyl groups. The results provide a strong molecular basis for tissue-specific targeting strategies, understanding drug-drug interactions as well as drug-metabolite interactions, and suggest a strategy for how drugs with comparable efficacy might be chosen in chronic liver or kidney disease (CKD) to minimize toxicity.

Unusual features and localization of the membrane kinome of Trypanosoma brucei

In many eukaryotes, multiple protein kinases are situated in the plasma membrane where they respond to extracellular ligands. Ligand binding elicits a signal that is transmitted across the membrane, leading to activation of the cytosolic kinase domain. Humans have over 100 receptor protein kinases. In contrast, our search of the Trypanosoma brucei kinome showed that there were only ten protein kinases with predicted transmembrane domains, and unlike other eukaryotic transmembrane kinases, seven are predicted to bear multiple transmembrane domains. Most of the ten kinases, including their transmembrane domains, are conserved in both Trypanosoma cruzi and Leishmania species. Several possess accessory domains, such as Kelch, nucleotide cyclase, and forkhead-associated domains. Surprisingly, two contain multiple regions with predicted structural similarity to domains in bacterial signaling proteins. A few of the protein kinases have previously been localized to subcellular structures such as endosomes or lipid bodies. We examined the localization of epitope-tagged versions of seven of the predicted transmembrane kinases in T. brucei bloodstream forms and show that five localized to the endoplasmic reticulum. The last two kinases are enzymatically active, integral membrane proteins associated with the flagellum, flagellar pocket, or adjacent structures as shown by both fluorescence and immunoelectron microscopy. Thus, these kinases are positioned in structures suggesting participation in signal transduction from the external environment.

Urate Transporters in the Kidney: What Clinicians Need to Know

Urate is produced in the liver by the degradation of purines from the diet and nucleotide turnover and excreted by the kidney and gut. The kidney is the major route of urate removal and has a pivotal role in the regulation of urate homeostasis. Approximately 10% of the glomerular filtered urate is excreted in the urine, and the remainder is reabsorbed by the proximal tubule. However, the transport of urate in the proximal tubule is bidirectional: reabsorption and secretion. Thus, an increase in reabsorption or a decrease in secretion may induce hyperuricemia. In contrast, a decrease in reabsorption or an increase in secretion may result in hyperuricosuria. In the proximal tubule, urate reabsorption is mainly mediated by apical URAT1 (SLC22A12) and basolateral GLUT9 (SLC2A9) transporter. OAT4 (SLC22A11) also acts in urate reabsorption in the apical membrane, and its polymorphism is associated with the risk of hyperuricemia. Renal hypouricemia is caused by SLC22A12 or SLC2A9 loss-of-function mutations, and it may be complicated by exercise-induced acute kidney injury. URAT1 and GLUT9 are also drug targets for uricosuric agents. Sodium-glucose cotransporter inhibitors may induce hyperuricosuria by inhibiting GLUT9b located in the apical plasma membrane. Urate secretion is mediated by basolateral OAT1 (SLC22A6) and OAT3 (SLC22A8) and apical ATP-binding cassette super-family G member 2 (ABCG2), NPT1 (SLC17A1), and NPT4 (SLC17A3) transporter in the proximal tubule. NPT1 and NPT4 may be key players in renal urate secretion in humans, and deletion of SLC22A6 and SLC22A8 in mice leads to decreased urate excretion. Dysfunctional variants of ABCG2 inhibit urate secretion from the gut and kidney and may cause gout. In summary, the net result of urate transport in the proximal tubule is determined by the dominance of transporters between reabsorption (URAT1, OAT4, and GLUT9) and secretion (ABCG2, NPT1, NPT4, OAT1, and OAT3).

Function of Uric Acid Transporters and Their Inhibitors in Hyperuricaemia

Disorders of uric acid metabolism may be associated with pathological processes in many diseases, including diabetes mellitus, cardiovascular disease, and kidney disease. These diseases can further promote uric acid accumulation in the body, leading to a vicious cycle. Preliminary studies have proven many mechanisms such as oxidative stress, lipid metabolism disorders, and rennin angiotensin axis involving in the progression of hyperuricaemia-related diseases. However, there is still lack of effective clinical treatment for hyperuricaemia. According to previous research results, NPT1, NPT4, OAT1, OAT2, OAT3, OAT4, URAT1, GLUT9, ABCG2, PDZK1, these urate transports are closely related to serum uric acid level. Targeting at urate transporters and urate-lowering drugs can enhance our understanding of hyperuricaemia and hyperuricaemia-related diseases. This review may put forward essential references or cross references to be contributed to further elucidate traditional and novel urate-lowering drugs benefits as well as provides theoretical support for the scientific research on hyperuricemia and related diseases.

Modulation of Urate Transport by Drugs

BACKGROUND

Serum urate (SU) levels in primates are extraordinarily high among mammals. Urate is a Janus-faced molecule that acts physiologically as a protective antioxidant but provokes inflammation and gout when it precipitates at high concentrations. Transporters play crucial roles in urate disposition, and drugs that interact with urate transporters either by intention or by accident may modulate SU levels. We examined whether in vitro transporter interaction studies may clarify and predict such effects.

METHODS

Transporter interaction profiles of clinically proven urate-lowering (uricosuric) and hyperuricemic drugs were compiled from the literature, and the predictive value of in vitro-derived cut-offs like C_max/IC₅₀ on the in vivo outcome (clinically relevant decrease or increase of SU) was assessed.

RESULTS

Interaction with the major reabsorptive urate transporter URAT1 appears to be dominant over interactions with secretory transporters in determining the net effect of a drug on SU levels. In vitro inhibition interpreted using the recommended cut-offs is useful at predicting the clinical outcome.

CONCLUSIONS

In vitro safety assessments regarding urate transport should be done early in drug development to identify candidates at risk of causing major imbalances. Attention should be paid both to the inhibition of secretory transporters and inhibition or trans-stimulation of reabsorptive transporters, especially URAT1.

Influence of ABC transporters on the excretion of ciprofloxacin assessed with PET imaging in mice

Ciprofloxacin is a commonly prescribed fluoroquinolone antibiotic which is cleared by active tubular secretion and intestinal excretion. Ciprofloxacin is a known substrate of the ATP-binding cassette (ABC) transporters breast cancer resistance protein (BCRP) and multidrug resistance-associated protein 4 (MRP4). In this work, we used positron emission tomography (PET) imaging to investigate the influence of BCRP, MRP4, MRP2 and P-glycoprotein (P-gp) on the excretion of [¹⁸F]ciprofloxacin in mice. Dynamic 90-min PET scans were performed after intravenous injection of [¹⁸F]ciprofloxacin in wild-type mice without and with pre-treatment with the broad-spectrum MRP inhibitor MK571. Moreover, [¹⁸F]ciprofloxacin PET scans were performed in Abcc4^(-/-), Abcc2^(-/-), Abcc4^(-/-)Abcg2^(-/-) and Abcb1a/b^(-/-)Abcg2^(-/-) mice. In addition to non-compartmental pharmacokinetic (PK) analysis, a novel three-compartment PK model was developed for a detailed assessment of the renal disposition of [¹⁸F]ciprofloxacin. In MK571 pre-treated mice, a significant increase in the blood exposure to [¹⁸F]ciprofloxacin was observed along with a significant reduction in the renal and intestinal clearances. PK modelling revealed a significant reduction in renal radioactivity uptake (CL₁) and in the rate constants for transfer of radioactivity from the corticomedullary renal region into blood (k₂) and urine (k₃), respectively, after MK571 administration. No changes in the renal clearance or in the estimated kidney PK model parameters were observed in any of the studied knockout models, while a significant reduction in the intestinal clearance was observed in Abcc2^(-/-) and Abcc4^(-/-)Abcg2^(-/-) mice. Our data failed to reveal a role of any of the studied ABC transporters in the tubular secretion of ciprofloxacin. This may indicate that ciprofloxacin is handled in the kidneys by more than one transporter family, most likely with a great degree of mutual functional redundancy. Our study highlights the potential of PET imaging for an assessment of transporter-mediated renal excretion of radiolabelled drugs.

Post-translational regulation of the major drug transporters in the families of organic anion transporters and organic anion-transporting polypeptides

The organic anion transporters (OATs) and organic anion-transporting polypeptides (OATPs) belong to the solute carrier (SLC) transporter superfamily and play important roles in handling various endogenous and exogenous compounds of anionic charge. The OATs and OATPs are often implicated in drug therapy by impacting the pharmacokinetics of clinically important drugs and, thereby, drug exposure in the target organs or cells. Various mechanisms (e.g. genetic, environmental, and disease-related factors, drug-drug interactions, and food-drug interactions) can lead to variations in the expression and activity of the anion drug-transporting proteins of OATs and OATPs, possibly impacting the therapeutic outcomes. Previous investigations mainly focused on the regulation at the transcriptional level and drug-drug interactions as competing substrates or inhibitors. Recently, evidence has accumulated that cellular trafficking, post-translational modification, and degradation mechanisms serve as another important layer for the mechanisms underlying the variations in the OATs and OATPs. This review will provide a brief overview of the major OATs and OATPs implicated in drug therapy and summarize recent progress in our understanding of the post-translational modifications, in particular ubiquitination and degradation pathways of the individual OATs and OATPs implicated in drug therapy.

Molecular Pathophysiology of Uric Acid Homeostasis

Uric acid, the end product of purine metabolism, plays a key role in the pathogenesis of gout and other disease processes. The circulating serum uric acid concentration is governed by the relative balance of hepatic production, intestinal secretion, and renal tubular reabsorption and secretion. An elegant synergy between genome-wide association studies and transport physiology has led to the identification and characterization of the major transporters involved with urate reabsorption and secretion, in both kidney and intestine. This development, combined with continued analysis of population-level genetic data, has yielded an increasingly refined mechanistic understanding of uric acid homeostasis as well as greater understanding of the genetic and acquired influences on serum uric acid concentration. The continued delineation of novel and established regulatory pathways that regulate uric acid homeostasis promises to lead to a more complete understanding of uric acid-associated diseases and to identify new targets for treatment.

Completing the Enalaprilat Excretion Pathway-Renal Handling by the Proximal Tubule

BACKGROUND

Enalapril is often used in the treatment of cardiovascular diseases. Clinical data suggest that the urinary excretion of enalaprilat, the active metabolite of enalapril, is mediated by renal transporters. We aimed to identify enalaprilat specificity for renal proximal tubular transporters.

METHODS

Baculovirus-transduced HEK293 cells overexpressing proximal tubular transporters were used to study enalaprilat cellular uptake. Uptake into cells overexpressing the basolateral transporters OCT2, OAT1, OAT2, or OAT3 and apical transporters OAT4, PEPT1, PEPT2, OCTN1, OCTN2, MATE1, MATE2k, and URAT1 was compared with mock-transduced control cells. Transport by renal efflux transporters MRP2, MPR4, P-gp, and BCRP was tested using a vesicular assay. Enalaprilat concentrations were measured using LC-MS/MS.

RESULTS

Uptake of enalaprilat into cells expressing OAT3 as well as OAT4 was significantly higher compared to control cells. The enalaprilat affinity for OAT3 was 640 (95% CI: 520-770) µM. For OAT4, no reliable affinity constant could be determined using concentrations up to 3 mM. No transport was observed for other transporters.

CONCLUSION

The affinity of enalaprilat for OAT3 and OAT4 was notably low compared to other substrates. Taking this affinity and clinically relevant plasma concentrations of enalaprilat and other OAT3 substrates into account, we believe that drug-drug interactions on a transporter level do not have a therapeutic consequence and will not require dose adjustments of enalaprilat itself or other OAT3 substrates.

[131I]MIBG exports via MRP transporters and inhibition of the MRP transporters improves accumulation of [131I]MIBG in neuroblastoma

INTRODUCTION

¹³¹I-labeled m-iodobenzylguanidine ([¹³¹I]MIBG) has been used to treat neuroblastoma patients, but [¹³¹I]MIBG may be immediately excreted from the cancer cells by the adenosine triphosphate binding cassette transporters, similar to anticancer drugs. The purpose of this study was to clarify the efflux mechanism of [¹³¹I]MIBG in neuroblastomas and improve accumulation by inhibition of the transporter in neuroblastomas.

METHODS

[¹³¹I]MIBG was incubated in human embryonic kidney (HEK)293 cells expressing human organic anion transporting polypeptide (OATP)1B1, OATP1B3, OATP2B1, organic anion transporter (OAT)1 and OAT2, organic cation transporter (OCT)1 and OCT2, and sodium taurocholate cotransporting polypeptide, and in vesicles expressing P-glycoprotein (MDR1), multidrug resistance associated protein (MRP)1-4, or breast cancer resistance protein with and without MK-571 and probenecid (MRP inhibitors). Time activity curves of [¹³¹I]MIBG with and without MK-571 and probenecid were established using an SK-N-SH neuroblastoma cell line, and transporter expression of multiple drug resistance was measured. Biodistribution and SPECT imaging examinations were conducted using [¹²³I]MIBG with and without probenecid in SK-N-SH-bearing mice.

RESULTS

[¹³¹I]MIBG uptake was significantly higher in OAT1, OAT2, OCT1, and OCT2 than in mock cells. Uptake via OCT1 and OCT2 was little inhibited by MK-571 and probenecid. [¹³¹I]MIBG uptake into vesicles that highly expressed MRP1 or MRP4 was significantly higher in ATP than in AMP, and these inhibitors restored uptake to levels similar to that in AMP. Examining the time activity curves for [¹³¹I]MIBG in SK-N-SH cells, higher expressions of MDR1, MRP1, MRP4, and MK-571, or probenecid loading produced significantly higher uptake than in control at most incubation times. The ratios of tumors to blood or muscle in SK-N-SH-bearing mice were significantly increased by probenecid loading in comparison with normal mice.

CONCLUSIONS

[¹³¹I]MIBG exports via MRP1 and MRP4 in neuroblastoma. The accumulation and tumor-to-blood or muscle ratios of [¹³¹I]MIBG are improved by inhibition of MRPs with probenecid in neuroblastoma. ADVANCES IN KNOWLEDGE: [¹³¹I]MIBG, widely used for treatment of neuroendocrine tumors including neuroblastoma, is excreted via MRP1 and MRP4 in neuroblastoma.

IMPLICATIONS FOR PATIENT CARE

Loading with probenecid, OAT, and MRP inhibitors improves [¹³¹I]MIBG accumulation.

Apoptotic and anti-proliferative effect of guanosine and guanosine derivatives in HuT-78 T lymphoma cells

The effects of 100 μM of 3',5'-cGMP, cAMP, cCMP, and cUMP as well as of the corresponding membrane-permeant acetoxymethyl esters on anti-CD3-antibody (OKT3)-induced IL-2 production of HuT-78 cutaneous T cell lymphoma (Sézary lymphoma) cells were analyzed. Only 3',5'-cGMP significantly reduced IL-2 production. Flow cytometric analysis of apoptotic (propidium iodide/annexin V staining) and anti-proliferative (CFSE staining) effects revealed that 3',5'-cGMP concentrations > 50 μM strongly inhibited proliferation and promoted apoptosis of HuT-78 cells (cultured in the presence of αCD3 antibody). Similar effects were observed for the positional isomer 2',3'-cGMP and for 2',-GMP, 3'-GMP, 5'-GMP, and guanosine. By contrast, guanosine and guanosine-derived nucleotides had no cytotoxic effect on peripheral blood mononuclear cells (PBMCs) or acute lymphocytic leukemia (ALL) xenograft cells. The anti-proliferative and apoptotic effects of guanosine and guanosine-derived compounds on HuT-78 cells were completely eliminated by the nucleoside transport inhibitor NBMPR (S-(4-Nitrobenzyl)-6-thioinosine). By contrast, the ecto-phosphodiesterase inhibitor DPSPX (1,3-dipropyl-8-sulfophenylxanthine) and the CD73 ecto-5'-nucleotidase inhibitor AMP-CP (adenosine 5'-(α,β-methylene)diphosphate) were not protective. We hypothesize that HuT-78 cells metabolize guanosine-derived nucleotides to guanosine by yet unknown mechanisms. Guanosine then enters the cells by an NBMPR-sensitive nucleoside transporter and exerts cytotoxic effects. This transporter may be ENT1 because NBMPR counteracted guanosine cytotoxicity in HuT-78 cells with nanomolar efficacy (IC50 of 25-30 nM). Future studies should further clarify the mechanism of the observed effects and address the question, whether guanosine or guanosine-derived nucleotides may serve as adjuvants in the therapy of cancers that express appropriate nucleoside transporters and are sensitive to established nucleoside-derived cytostatic drugs.

Metabolic Acidosis Alters Expression of Slc22 Transporters in Mouse Kidney

INTRODUCTION

The kidneys play a central role in eliminating metabolic waste products and drugs through transporter-mediated excretion along the proximal tubule. This task is mostly achieved through a variety of transporters from the solute carrier family 22 (SLC22) family of organic cation and anion transporters. Metabolic acidosis modulates metabolic and renal functions and also affects the clearance of metabolites and drugs from the body. We had previously shown that induction of metabolic acidosis in mice alters a large set of transcripts, among them also many transporters including transporters from the Slc22 family.

OBJECTIVE

Here we further investigated the impact of acidosis on Slc22 family members.

METHODS

Metabolic acidosis was induced for 2 or 7 days with NH4Cl, some animals also received the uricase inhibitor oxonic acid for comparison. Expression of transporters was studied by qPCR and immunoblotting.

RESULTS

NH4Cl induced no significant changes in plasma or urine uric acid levels but caused downregulation of Slc22a1 (Oct1), Slc22a6 (Oat1), Slc22a19 (Oat5), and -Slc22a12 (Urat1) at mRNA level. In contrast, Slc22a4 mRNA (Octn1) was upregulated. On protein level, NH4Cl increased Octn1 (after 7 days) and Urat1 (after 2 days) abundance and decreased Oat1 (after 2 days) and Urat1 (after 7 days). Oxonic acid had no impact on protein abundance of any of the transporters tested.

CONCLUSION

In summary, metabolic acidosis alters expression of several transporters involved in renal excretion of metabolic waste products and drugs. This may have implications for drug kinetics and clearance of waste metabolites.

Nicotinic acid transport into human liver involves organic anion transporter 2 (SLC22A7)

Nicotinic acid (NA) and nicotinamide (NAM) are biosynthetic precursors of nicotinamide adenine dinucleotide (NAD⁺) - a physiologically important coenzyme that maintains the redox state of cells. Mechanisms driving their entry into cells are not well understood. Here we evaluated the hepatic uptake mechanism(s) of NA and NAM using transporter-transfected cell systems and primary human hepatocytes. NA showed robust organic anion transporter (OAT)2-mediated transport with an uptake ratio (i.e., ratio of accumulation in transfect cells to wild-type cells) of 9.7 ± 0.3, and a Michaelis-Menten constant (K_m) of 13.5 ± 3.3 µM. However, no transport was apparent via other major hepatic uptake and renal secretory transporters, including OAT1/3/4, organic anion transporting polypeptide (OATP)1B1/1B3/2B1, sodium-taurocholate co-transporting polypeptide, organ cation transporter 1/2/3. OAT2-specific transport of NA was inhibited by ketoprofen and indomethacin (known OAT2 inhibitors) in a concentration-dependent manner. Similarly, NA uptake into primary human hepatocytes showed pH- and concentration-dependence and was subject to inhibition by specific OAT2 inhibitors. Unlike NA, NAM was not transported by the hepatic and renal solute carriers upon assessment in transfected cells, although its uptake into human hepatocytes was significantly inhibited by excess unlabelled NAM and a pan-SLC inhibitor (rifamycin SV 1 mM). In conclusion, these studies demonstrate, for the first time, a specific transport mechanism for NA uptake in the human liver and suggest that OAT2 (SLC22A7) has a critical role in its physiological and pharmacological functions.

3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF) prevents high fat diet-induced insulin resistance via maintenance of hepatic lipid homeostasis

AIM

Omega-3 fatty acid ethyl ester supplements, available by prescription, are common in the treatment of dyslipidaemia in humans. Recent studies show that 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF), a metabolite formed from fish oil supplementation, was able to prevent and reverse high fat diet (HFD)-induced fatty liver in mice. In the present study, we investigated the underlying molecular mechanisms responsible for CMPF's hepatic lipid-lowering effects.

MATERIALS AND METHODS

CD1 male mice were i.p. injected with CMPF (dosage, 6 mg/kg) for 7 days, followed by 5 weeks of a 60% HFD to induce a fatty liver phenotype. Metabolic parameters, liver morphology, lipid content, protein expression and microarray analysis were assessed. We also utilized primary hepatocytes, an in vitro model, to further investigate the direct effects of CMPF on hepatic lipid utilization and biosynthesis.

RESULTS

CMPF-treated mice display enhanced hepatic lipid clearance while hepatic lipid storage is prevented, thereby protecting against liver lipid accumulation and development of HFD-induced hepatic insulin resistance. Mechanistically, as CMPF enters the liver, it acts as an allosteric acetyl-coA carboxylase (ACC) inhibitor, which directly induces both fatty acid oxidation and hepatic production of fibroblast growth factor 21 (FGF21). A feed-back loop is initiated by CMPF, which exists between ACC inhibition, fatty acid oxidation and production of FGF21. As a consequence, an adaptive decrease in Insig2/SREBP-1c/FAS protein expression results in priming of the liver to prevent a HFD-induced fatty liver phenotype.

CONCLUSION

CMPF is a potential driver of hepatic lipid metabolism, preventing diet-induced hepatic lipid deposition and insulin resistance in the long term.

Genetics of guanylyl cyclase pathways in the cochlea and their influence on hearing

Although hearing loss is the most common sensory deficit in Western societies, there are no successful pharmacological treatments for this disorder. Recent experiments have demonstrated that manipulation of intracellular cyclic guanosine monophosphate (cGMP) concentrations can have both beneficial and harmful effects on hearing. In this review, we will examine the role of cGMP as a key second messenger involved in many aspects of cochlear function and discuss the known functions of downstream effectors of cGMP in sound processing. The nitric oxide-stimulated soluble guanylyl cyclase system (sGC) and the two natriuretic peptide-stimulated particulate GCs (pGCs) will be more extensively covered because they have been studied most thoroughly. The cochlear GC systems are attractive targets for medical interventions that improve hearing while simultaneously representing an under investigated source of sensorineural hearing loss.

The role of OAT2 (SLC22A7) in the cyclic nucleotide biokinetics of human erythrocytes

The present study was conducted to characterise the transporter(s) responsible for the uptake of cyclic nucleotides to human erythrocytes. Western blotting showed that hRBC expressed OAT2 (SLC22A7), but detection of OAT1 (SLC22A6), or OAT3 (SLC22A8) was not possible. Intact hRBC were employed to clarify the simultaneous cyclic nucleotide egression and uptake. Both these opposing processes were studied. The K_m‐values for high affinity efflux was 3.5 ± 0.1 and 39.4 ± 5.7 μM for cGMP and cAMP, respectively. The respective values for low affinity efflux were 212 ± 11 and 339 ± 42 μM. The uptake was characterised with apparently low affinity and similar K_m‐values for cGMP (2.2 mM) and cAMP (0.89 mM). Using an iterative approach in order to balance uptake with efflux, the predicted real K_m‐values for uptake were 100–200 μM for cGMP and 50–150 μM for cAMP. The established OAT2‐substrate indomethacin showed a competitive interaction with cyclic nucleotide uptake. Creatinine, also an OAT2 substrate, showed saturable uptake with a K_m of 854 ± 98 μM. Unexpectedly, co‐incubation with cyclic nucleotides showed an uncompetitive inhibition. The observed K_m‐values were 399 ± 44 and 259 ± 30 μM for creatinine, in the presence of cGMP and cAMP, respectively. Finally, the OAT1‐substrate para‐aminohippurate (PAH) showed some uptake (K_m‐value of 2.0 ± 0.4 mM) but did not interact with cyclic nucleotide or indomethacin transport.

In vitro characterization of zebrafish (Danio rerio) organic anion transporters Oat2a-e

OATS/Oats are transmembrane proteins that transport a variety of drugs, environmental toxins and endogenous metabolites into the cell. Zebrafish (Danio rerio) has seven OAT orthologs: Oat1, Oat2a-e and Oat3. In this study we specifically address Oat2 (Slc22a7) family. Conserved synteny analysis showed localization of zebrafish oat2 genes on two chromosomes, 11 and 17. All five zebrafish Oats were localized by live cell imaging in membranes of transiently transfected HEK293-T cells, and Oat2a, b, d, and e were confirmed using western blot analysis. Functional studies using the HEK293T cells overexpressing zebrafish Oats revealed two model fluorescent substrates of three Oats: Lucifer yellow for Oat2a and Oat2d (Km 122, and 49.7μM), and 6-carboxyfluorescein for Oat2b and Oat2d (Km 199.7, and 266.9μM). The initial screening of a series of diverse endo- and xenobiotics showed interaction with a number of compounds, including cGMP and diclofenac (IC50 27.74, and 19.14μM) with Oat2a; estrone-3-sulfate and diclofenac (IC50 30.96, and 12.6μM) with Oat2b; and fumarate and indomethacin (IC50 68.24, and 20.41μM) with Oat2d. This study provides the first comprehensive data set on Oat2 in zebrafish and offers an important basis for more detailed molecular and (eco)toxicological characterizations of these transporters.

Differential Interaction of Dantrolene, Glafenine, Nalidixic Acid, and Prazosin with Human Organic Anion Transporters 1 and 3

In renal proximal tubule cells, the organic anion transporters 1 and 3 (OAT1 and OAT3) in the basolateral membrane and the multidrug resistance-associated protein 4 (MRP4) in the apical membrane share substrates and co-operate in renal drug secretion. We hypothesized that recently identified MRP4 inhibitors dantrolene, glafenine, nalidixic acid, and prazosin also interact with human OAT1 and/or OAT3 stably transfected in human embryonic kidney 293 cells. These four drugs were tested as possible inhibitors of p-[³H]aminohippurate (PAH) and [¹⁴C]glutarate uptake by OAT1, and of [³H]estrone-3-sulfate (ES) uptake by OAT3. In addition, we explored whether these drugs decrease the equilibrium distribution of radiolabeled PAH, glutarate, or ES, an approach intended to indirectly suggest drug/substrate exchange through OAT1 and OAT3. With OAT3, a dose-dependent inhibition of [³H]ES uptake and a downward shift in [³H]ES equilibrium were observed, indicating that all four drugs bind to OAT3 and may possibly be translocated. In contrast, the interaction with OAT1 was more complex. With [¹⁴C]glutarate as substrate, all four drugs inhibited uptake but only glafenine and nalidixic acid shifted glutarate equilibrium. Using [³H]PAH as a substrate of OAT1, nalidixic acid inhibited but dantrolene, glafenine, and prazosin stimulated uptake. Nalidixic acid decreased equilibrium content of [³H]PAH, suggesting that it may possibly be exchanged by OAT1. Taken together, OAT1 and OAT3 interact with the MRP4 inhibitors dantrolene, glafenine, nalidixic acid, and prazosin, indicating overlapping specificities. At OAT1, more than one binding site must be assumed to explain substrate and drug-dependent stimulation and inhibition of transport activity.

Organic Anion Transporter 2: An Enigmatic Human Solute Carrier

As a member of the solute carrier 22A (SLC22A) family, organic anion transporter 2 (OAT2; SLC22A7) is emerging as an important drug transporter because of its expression in both the liver and kidney, two major eliminating organs, and its ability to transport not only a wide variety of xenobiotics but also numerous physiologically important endogenous compounds, like creatinine and cGMP. However, OAT2 has received relatively little attention compared with other OATs and solute carriers (SLCs), like organic cation transporters, sodium-dependent taurocholate cotransporting polypeptide, multidrug and toxin extrusion proteins, and organic anion-transporting polypeptides. Overall, the literature describing OAT2 is rapidly evolving, with numerous publications contradicting each other regarding the transport mechanism, tissue distribution, and transport of creatinine and cGMP, two important endogenous OAT2 substrates. Despite its status as a liver and kidney SLC, tools for assessing its activity and inhibition are lacking, and its role in drug disposition and elimination remains to be defined. The current review focuses on the available and emerging literature describing OAT2. We envision that OAT2 will gain more prominence as its expression, substrate, and inhibitor profile is investigated further and compared with other SLCs.

Mechanisms involved in the transport of mercuric ions in target tissues

Mercury exists in the environment in various forms, all of which pose a risk to human health. Despite guidelines regulating the industrial release of mercury into the environment, humans continue to be exposed regularly to various forms of this metal via inhalation or ingestion. Following exposure, mercuric ions are taken up by and accumulate in numerous organs, including brain, intestine, kidney, liver, and placenta. In order to understand the toxicological effects of exposure to mercury, a thorough understanding of the mechanisms that facilitate entry of mercuric ions into target cells must first be obtained. A number of mechanisms for the transport of mercuric ions into target cells and organs have been proposed in recent years. However, the ability of these mechanisms to transport mercuric ions and the regulatory features of these carriers have not been characterized completely. The purpose of this review is to summarize the current findings related to the mechanisms that may be involved in the transport of inorganic and organic forms of mercury in target tissues and organs. This review will describe mechanisms known to be involved in the transport of mercury and will also propose additional mechanisms that may potentially be involved in the transport of mercuric ions into target cells.

Distribution of organic anion transporters NaDC3 and OAT1-3 along the human nephron

The initial step in renal secretion of organic anions (OAs) is mediated by transporters in the basolateral membrane (BLM). Contributors to this process are primary active Na(+)-K(+)-ATPase (EC 3.6.3.9), secondary active Na(+)-dicarboxylate cotransporter 3 (NaDC3/SLC13A3), and tertiary active OA transporters (OATs) OAT1/SLC22A6, OAT2/SLC22A7, and OAT3/SLC22A8. In human kidneys, we analyzed the localization of these transporters by immunochemical methods in tissue cryosections and isolated membranes. The specificity of antibodies was validated with human embryonic kidney-293 cells stably transfected with functional OATs. Na(+)-K(+)-ATPase was immunolocalized to the BLM along the entire human nephron. NaDC3-related immunostaining was detected in the BLM of proximal tubules and in the BLM and/or luminal membrane of principal cells in connecting segments and collecting ducts. The thin and thick ascending limbs, macula densa, and distal tubules exhibited no reactivity with the anti-NaDC3 antibody. OAT1-OAT3-related immunostaining in human kidneys was detected only in the BLM of cortical proximal tubules; all three OATs were stained more intensely in S1/S2 segments compared with S3 segment in medullary rays, whereas the S3 segment in the outer stripe remained unstained. Expression of NaDC3, OAT1, OAT2, and OAT3 proteins exhibited considerable interindividual variability in both male and female kidneys, and sex differences in their expression could not be detected. Our experiments provide a side-by-side comparison of basolateral transporters cooperating in renal OA secretion in the human kidney.